Multilayer composites and manufacture of same

ABSTRACT

The present invention is directed towards a process of depositing multilayer thin films, disk-shaped targets for deposition of multilayer thin films by a pulsed laser or pulsed electron beam deposition process, where the disk-shaped targets include at least two segments with differing compositions, and a multilayer thin film structure having alternating layers of a first composition and a second composition, a pair of the alternating layers defining a bi-layer wherein the thin film structure includes at least 20 bi-layers per micron of thin film such that an individual bi-layer has a thickness of less than about 100 nanometers.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to a process and targets for thecontrolled deposition of multilayer films, e.g., multilayer hightemperature superconducting (HTS) films, films having functionallygraded compositions, e.g., HTS films having functionally gradedcompositions, and films doped with minor amounts of a second material,e.g., HTS films doped with minor amounts of a second material.

BACKGROUND OF THE INVENTION

One conventional process for the deposition of superconducting thickfilms, such as YBCO, and other industrial films such as semi-conductorfilms, ferroelectric films, insulating or optical coating films, and thelike, is pulsed laser deposition (PLD). In such a process, a target,typically a disk-like shaped target, of the material or materials to bedeposited is contacted with a laser beam of the desired energy andfrequency. Commonly, such a disk-like target is rotated during theprocess to avoid contacting only a single spot of the target. In somePLD processes, a laser beam is simply rastered across sections of atarget so that it is the laser beam that is moved rather than thetarget.

Since initial development, coated conductor research on HTSsuperconductors has focused on fabricating increasing lengths of thematerial, while increasing the overall critical current carryingcapacity. Different research groups have developed several techniques offabricating coated conductors. Regardless of which techniques are usedfor the coated conductors, the goal of obtaining highly texturedsuperconducting thick films, such as YBa₂Cu₃O_(7-x) (YBCO), with highsupercurrent carrying capability on metal substrates remains. The use ofthick superconducting films for coated conductors appears logicalbecause both the total critical current and the engineering criticalcurrent density (defined as the ratio of total critical current and thecross-sectional area of the tape) are directly correlated with thethickness of the superconducting films. Multilayer HTS films haverecently been shown to yield high current superconducting compositesbecause high quality, thick HTS coatings can be grown with multilayers.

U.S. Pat. Nos. 5,356,522 and 5,580,667 by Lai et al. describe the use ofsectored targets in the preparation of thin film magnetic disks. Theirsectored targets are designed for deposition via sputtering as thetarget moves consecutively linearly through successive regions of thesputtering system. They do not describe sectored disks, do not describerotation of sectored targets during deposition, and do not describedeposition of high temperature superconducting materials.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention provides a processof depositing multilayer thin films by rotating a single target havingat least two segments with differing compositions under a processingbeam to generate processed material from the single target fordeposition of the processed material upon a substrate, the processingbeam contacting the segments with differing compositions in a controlleddefined manner, and contacting the processed material from the singletarget with the substrate under conditions sufficient to deposit theprocessed material upon the substrate, where processed material from thesegments with differing compositions is deposited in a predetermineddefined manner as a multilayer thin film. The segment compositions canbe single component or multicomponent materials.

In another embodiment, the present invention provides a process ofdepositing multilayer thin films by contacting a single target having atleast two segments with differing compositions under a processing beamin a controlled defined manner thereby generating processed materialfrom the single target for deposition of the processed material upon asubstrate, and contacting the processed material from the single targetwith the substrate under conditions sufficient to deposit the processedmaterial upon the substrate, where processed material from the segmentswith differing compositions is deposited in a predetermined definedmanner as a multilayer thin film. The segment compositions can be singlecomponent or multicomponent materials.

Further, the present invention provides a disk-shaped target fordeposition of multilayer thin films by a pulsed laser or pulsed electronbeam deposition process, such a disk-shaped target including at leasttwo segments with differing compositions. The segments can be singlecomponent or multicomponent materials.

Further, the present invention provides a multilayer thin film structurehaving alternating layers of a first composition and a secondcomposition, a pair of the alternating layers defining a bi-layerwherein the thin film structure includes at least 20 bi-layers permicron of thin film such that an individual bi-layer has a thickness ofless than about 50 nanometers. In another embodiment, the alternatinglayers can include more than two compositionally different layers suchthat a tri-layer, quad-layer or the like is defined and the thin filmstructure can include a large multiple of such tri-layers, quad-layersor the like per micron of thin film.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1(a)-(i) show exemplary configurations for targets in accordancewith the present invention.

FIG. 2 shows a film structure obtainable with a sectored target whendeposition parameters are varied during deposition in accordance withthe present invention.

FIG. 3 shows a plot of field dependent measurements of superconductingproperties of various multilayer films produced in accordance with thepresent invention.

DETAILED DESCRIPTION

The present invention is concerned with targets and a process for thepreparation of multilayer films, e.g., high temperature superconducting(HTS) films, films having functionally graded compositions, e.g., HTSfilms having functionally graded compositions, and films doped withminor amounts of a second material, e.g., HTS films doped with minoramounts of a second material. The applications of the present inventionare widespread. Not only is it very applicable to the superconductorindustry, but also of interest to other film-related industries forfilms such as semiconductors, ferroelectrics, magnetic coatings,magnetoresistance materials, thermoelectrics, insulators, opticalcoatings and the like. Multilayer structures with repeating layers havebeen previously described for magnetic films of, e.g., Pt/Co, PdCo andthe like and for such films using intermediate insulating layers of SiO₂and the like, for giant magnetoresistance structures of, e.g.,alternating ferromagnetic and non-magnetic layers, for thermoelectricmaterials such as trilayer structures of repeating layers of PbTe,PbSeTe and Te and the like, and semiconductor structures of, e.g.,repeating trilayers of InAs, GaSb and AlSb and the like. Each suchprevious structure may be prepared using the process and sectored targetof the present invention by properly designing the target and process.

The present invention allows the growth of high-density multilayerstructures sometimes referred to as superlattice-like structures. Theterm “superlattice structure” refers to a composite structure made ofalternating ultrathin layers of different component materials. Asuperlattice structure typically has an energy band structure which isdifferent than, but related to, the energy band structures of itscomponent materials. The selection of the component materials of asuperlattice structure, and the addition of relative amounts of thosecomponent materials, will primarily determine the resulting propertiesof a superlattice structure as well as whether, and by how much, thoseproperties will differ from those of the individual component materialsa superlattice structure.

The process of the present invention can allow preparation of multilayercomposites with a wide range of thicknesses with from a single unit (ofalternating layers of the different deposited materials, e.g., abi-layer of a first composition and a second composition) up to manyunits with total combined thicknesses greater than, e.g., one micron.

The targets and process of the present invention allow the use of only asingle pulsed laser deposition (PLD) target in the preparation ofmultilayer films, e.g., multilayer HTS films. A target is formed priorto use to contain one or more additional sectors, regions, or othershapes that have a different composition of material relative to theprimary matrix of the target as shown in FIGS. 1(a)-(i). Due to thesimplistic design and easy use in existing PLD systems, the presentinvention offers significant advantages in terms of composition andstructural control that are not readily accessible by other processes.

The HTS composites are, in the broadest sense, composed of a substrate,possibly one or more buffer layers, and an HTS film, which is thefunctional object of the composite. The substrates can be single crystalsubstrates such as strontium titanate (STO) or yttria-stabilizedzirconia (YSZ), textured polycrystalline substrates such asroll-textured nickel (RABiTS), or non-textured polycrystallinesubstrates that have a textured template film deposited on the surfacesuch as an ion-beam-assist deposited YSZ or MgO film on a nickel alloy,e.g., a nickel-chromium alloy. Often, but not always, buffer layers areemployed to facilitate the deposition of a final HTS layer. Examples ofbuffer materials can include cerium oxide, strontium titanate, strontiumruthenate, yttrium oxide, and lanthanum manganate (LaMnO₃). The finallayer can be a film or composite film that contains a desired HTSmaterial such as YBCO (Y-123).

The substrates can be other materials for other applications such assemiconductors, ferroelectrics, magnetic coatings, magnetoresistancematerials, thermoelectrics, insulators, optical coatings and the like.For example, for ferroelectrics, suitable substrates can includesilicon, platinum-coated silicon and other conductive material-coatedsilicon. For semiconductors, suitable substrates can include stainlesssteel, molybdenum and silicon. For magnetic coatings, suitablesubstrates can include silicon. For magnetoresistance materials,suitable substrates can include nonmagnetic materials such as glass,silicon, aluminum oxide (Al₂O₃), titanium carbide (TiC), silicon carbide(SiC), a sintered product of aluminum oxide and TiO, or ferrite. Forthermoelectrics, suitable substrates can include highly insulatingsilicon or silicon on an insulator (SOI).

The factors of pulsed laser deposition (PLD) that are important in thepractice of the present invention to form desired structures include thetarget rotation speed, pulse rate, pulse energy, and distance from thetarget center to the point on the target where the laser beam isincident. Variations in these parameters in conjunction with speciallydesigned targets can affect the periodicity and compositional makeup ofthe resulting film. These variations can be made between runs or changedduring film deposition in either a stepwise or continuous manner.

Similarly, the factors of pulsed electron beam deposition (PEBD) thatare important in the practice of the present invention to form desiredstructures include the target rotation speed, pulse rate, pulse energy,and distance from the target center to the point on the target where theelectron beam is incident. Variations in these parameters in conjunctionwith specially designed targets can affect the periodicity andcompositional makeup of the resulting film. These variations can be madebetween runs or changed during film deposition in either a stepwise orcontinuous manner.

The design of an individual target can allow an additional manner offilm deposition control. Examples of these targets are shown in FIGS.1(a)-(i). FIGS. 1(a)-(c) show pie-shaped sectors that comprise adesigned portion of the target. The fraction each sector or sectorscomprise of the target can be varied in a continuous manner dependingupon the needs of the intended final product. The sectored target isuseful in making multilayer films where periodicity is determined by therotation speed of the target, pulse rate, and energy of the laser.Changes in periodicity within a given deposition can be obtained byvarying in a stepwise or continuous manner the target rotation speed,laser pulse rate and laser energy. An example of the change in structureor periodicity is shown in FIG. 2. Functionally graded materials can beobtained by simply changing the rotation rate of the target in acontinuous manner during a specific deposition run. Initial rotationrate settings can produce the periodicity in multilayers shown at 20 inFIG. 2. Simply by slowing the rotation rate, the periodicity inmultilayers can be thicker as shown at 22. Changing back to the originalrate settings can again produce the periodicity in multilayers shown at24 the same as the original periodicity shown at 20. By varying thelaser rate and/or the target rotation, the resultant multilayer thinfilm can have a continuously varying periodicity. Such a periodicitycould gradually go from thinner layers to thicker layers, from thickerlayers to thinner layers, or many other possible configurations.

Other target designs are shown in FIGS. 1(d)-(i) and can be used to makeperiodic structures of perform controlled deposition of second phaseparticles within a film, e.g., an HTS film. Since the one or moremodified sectors of the target are not pie shaped in these designs, thedistance from the center of the target where the laser is incident nowbecomes an additional parameter that can be changed in a continuousmanner to affect the composition and structure of the resulting film,e.g., a HTS film.

Structures such as shown in FIGS. 1(h) and (i) would allow an operatorto switch between materials in a given run without having to switchtargets. For example, the target shown in FIG. 1(h) could be comprisedof a buffer layer material for the inner circle surrounded by an HTSmaterial. The same could be said of FIG. 1(i) except that now amultilayer structure could be formed in either the buffer layer or theHTS film. The examples discussed here demonstrate the wide range ofpossibilities available using a sectored target. The differing segmentcompositions for superconducting applications can employ variouscombinations of rare-earth-barium-copper oxides (RE-BCO) for thedifferent layers of a resultant multilayer superconductive structure.The rare earth metals can generally be any suitable rare earth metalfrom the periodic table, but are preferably chosen from among yttrium,neodymium, samarium, europium, gadolinium, erbium, dysprosium andytterbium. In a multilayer example, combinations for a first and thirdlayers (with an interlayer of insulating, conducting or superconductingmaterial) may include, for example, both layers of one mixed rare earthoxide combination, or one mixed rare earth oxide combination in thefirst layer and a different mixed rare earth oxide combination in thethird layer. For multilayer composites with more than three layers, thepossible mixture combinations would multiply but can readily be workedout by one skilled in the art. Yttrium is a preferred rare earth toinclude in forming the mixed rare earth oxide combinations.

In other applications such as semiconductors, ferroelectrics, magneticcoatings, ferromagnetic or magnetoresistance materials, thermoelectrics,insulators and the like, the differing materials for the segmentedcompositions are selected for the particular application. For example,for ferroelectrics, suitable segmented compositions can be of, e.g.,strontium titanate, barium titanate, lead zirconium titanate (PZT) andbarium titanate. For semiconductors, suitable segmented compositions canbe of, e.g., gallium arsenide (GaAs), indium arsenide (InAs), galliumantimonide (GaSb), indium phosphide (InP), lead telluride (PbTe),gallium nitride (GaN), gallium phosphide (GaP), aluminum antimonide(AlSb) and the like. For magnetic coatings, suitable segmentedcompositions can be of platinum and cobalt, palladium and cobalt,terbium and iron and the like. For magnetoresistance materials, suitablesegmented compositions can be of lanthanum strontium manganate(La_(0.7)Sr_(0.3)MnO₃), neodymium strontium manganate(Nd_(0.7)Sr_(0.3)MnO₃), lanthanum calcium manganate(La_(0.7)Ca_(0.3)MnO₃), lanthanum manganate (LaMnO₃), and the like. Forthermoelectrics, suitable segmented compositions can be, e.g., oflead-telluride (PbTe), lead-selenide-telluride (PbSeTe) and tellurium(Te).

The targets used in the examples were manufactured by traditional bulksintering techniques. In one embodiment, bulk superconducting powderswere manufactured separately by mechanical milling in isopropanol,drying, and then calcinating at 900° C. for 25 hours.

Targets were formed by forming a pie-shaped piece of metal to fit insidea disk-shaped die (2-inch diameter of a circular shape). A firstmaterial powder was loaded into the pie-shaped piece of metal while asecond material powder was loaded around the remainder of the die. Thefirst material powder can comprise as little or as much of the overalltarget volume as desired. The metal form can then be removed and thetarget pressed at 15 kilograms per square inch (kpsi) for a few seconds.The resultant segmented target can then be removed from the die andsintered in an oven to fully form the individual superconductingmaterials (for the superconducting embodiment) and to bond the first andsecond materials into a solid target. The target was ramped at 4° C. perminute to 900° C. and held for 25 hours in an oxygen atmosphere. It wasthen ramped down to 400° C. and held for 25 hours, ramped back up to925° C. and held for 25 hours, then ramped down to 400° C. and held foran additional 75 hours. After the latter step, the sample was allowed tofurnace cool (i.e., cool down by simply turning off the furnace) to roomtemperature. Such heating stages are not necessary for every typematerial that can be used as a material of a segment composition.

A film was deposited upon a STO substrate using the above target. Thefilm thickness was about 5000 Angstroms and the T_(c) was 92 K. Themeasured J_(c) of the film was 4×10⁶ amperes per square centimeter(A/cm²) at 75.5 K. The structure of the film consisted of a high-densityarrangement of multilayers. The periodicity of the bi-layer structurewas less than 20 nm. The number of individual layers, Y-123 and Eu-123,per micron exceeded 140. The field dependence of the superconductingproperties of the film is shown at 30 in FIG. 3. The properties werefound to be as good as some of the best single component YBCO films thathave been made in the same laboratory and shown at 32, 34 and 36.

Other methods of making, e.g., the multilayer structures are to useindividual targets that are then interchanged to make the differentlayers. However, this is somewhat labor intensive and not practical formaking the ultrafine multilayers as described by the present invention.Another method of making multilayers is described in a prior LANL patentwhere a mixed rare-earth superconducting film is deposited andsubsequently post-annealed to produce a layered structure due tosolubility instability and film segregation into different phases andmultiple layers. However, this approach is limited to certain materialsthat exhibit a thermodynamic instability and segregate into the twodifferent phases with changes in annealing conditions. In contrast, thepresent invention is limited only to the extent that the materials putinto the target do not significantly react with one another during thefinal sintering step during preparation of a robust target.

The present invention is seen as having applications in terms of addinga discrete second phase in the superconducting film. Having the secondphase as a discrete section of a target results in the PLD systemputting selected material at a regular interval onto the substrate thathas the stoichiometry only of the second phase. Uniformly mixing thissecond phase into the target would not accomplish this result.

The process and targets of the present invention are also of interest toother film deposition techniques where a target is employed such as insputtering. When sputtering, different materials typically havedifferent sputtering rates. With a sectored target of the presentinvention, only one source or target would be needed which simplifiesdesign and reduces costs for any deposition system. The sector or othershape within the target would be changed to account for differentsputtering rates for different materials and to tailor the compositionto the desired values. In this manner, only one sputtering target andgun would be needed.

The present invention is more particularly described in the followingexamples which are intended as illustrative only, since numerousmodifications and variations will be apparent to those skilled in theart.

EXAMPLE 1

Bulk superconducting powders of Y_(1.015)Ba₂Cu₃O_(y) (Y-123) andEu_(1.015)Ba₂Cu₃O_(y) (Eu-123) were manufactured separately bymechanical milling in isopropanol, drying, and then calcinating at 900°C. for 25 hours. A pie-shaped piece of metal was then formed to fitinside a 2-inch diameter die. The Eu-123 powder was loaded into thepie-shaped piece of metal while the Y-123 powder was loaded around theremainder of the 2-inch die. In this example, the Eu-123 powdercomprised approximately ⅙ of the overall target volume. The metal wasremoved and the target pressed at 15,000 pounds per square inch (psi)for a few seconds. The target was removed from the die and then sinteredin an oven to fully form the individual superconducting materials and tobond the materials into a solid target. The target was ramped at 4° C.per minute to 900° C. and held for 25 hours in an oxygen atmosphere. Itwas then ramped down to 400° C. and held for 25 hours, ramped back up to925° C. and held for 25 hours, then ramped down to 400° C. and held foran additional 75 hours. After the latter step, the sample was allowed tofurnace cool (i.e., cool down by simply turning off the furnace) to roomtemperature.

EXAMPLE 2

A film was deposited upon a STO substrate using the target fromExample 1. The film thickness was about 5000 Angstroms and the T_(c) was92 K. The measured J_(c) of the film was 4×10⁶ amperes per squarecentimeter (A/cm²). The structure of the film consisted of ahigh-density arrangement of multilayers. The periodicity of the bi-layerstructure was less than 20 nm. The number of individual layers, Y-123and Eu-123, per micron exceeded 140. The number of bi-layer pairs (70)translates into a periodicity of less than 20 nm for every pair ofalternating layers. Hence, controlled ultra-fine microstructuralfeatures in an HTS composite structure can be obtained. The fielddependence of the superconducting properties of the film is shown inFIG. 3. The properties were found to be as good as some of the bestsingle component YBCO films that have been made in the same laboratory.

EXAMPLE 3

Powders of Y-123 and Sm_(1.015)Ba₂Cu₃O_(y) (Sm-123) were used to maketwo targets in a similar manner to the Y/Eu target of Example 1. In thefirst of these targets, the Sm-123 powder comprised about ⅙ of thetarget with the balance made up of the Y-123 powder. In the second ofthese targets, the Y-123 powder comprised about ⅙ of the target with thebalance made up of the Sm-123 powder. Films were made on IBAD-YSZ coatedHastelloy metal substrates. An intervening layer of CeO₂ was depositedprior to using the sectored target. In the case where the Sm-123 made up⅙ of the target, a film with a T_(c) of 92.4 K and an average J_(c)value from microbridge measurements of 0.775×10⁶ A/cm² was obtained. Inthe other film made where the Y-123 made up ⅙ of the target, a film witha T_(c) of 92.4 K and an average J_(c) value from microbridgemeasurements of 1.2×10⁶ A/cm² was obtained.

EXAMPLE 4

A sectored target similar to that shown in FIG. 1(d) was alsofabricated. In that instance, a Gd₂BaCuO_(y) (Gd-211) powder was used tomake a sector and Y-123 powder made up the remainder of the target. Tofabricate this target, the Gd-211 powder was first put into a silversheath and pressed in a rectangular die to fabricate a rectangularshaped sector for the target. This piece was then placed in the 2-inchdie and the Y-123 powder was filled in around it. The target was thenpressed together at 15 kspi and then sintered as before. A thin film wasdeposited upon a STO substrate using this target. The T_(c) of the filmwas 90.8 K. The J_(c) of the film was at least 1.6×10⁶ A/cm² at liquidnitrogen temperatures. There was some problem in measuring the actualthickness of the film such that the J_(c) value was considered aconservative estimate.

EXAMPLE 5

Bulk superconducting powders of Dy_(1.015)Ba₂Cu₃O_(y) (Dy-123) andEu_(1.015)Ba₂Cu₃O_(y) (Eu-123) were manufactured separately bymechanical milling in isopropanol, drying, and then calcinating at 900°C. for 25 hours. A pie-shaped piece of metal was then formed to fitinside a 2-inch diameter die. The Eu-123 powder was loaded into thepie-shaped piece of metal while the Dy-123 powder was loaded around theremainder of the 2-inch die. In this example, the Eu-123 powdercomprised approximately ⅓ of the overall target volume. The metal wasremoved and the target pressed at 15 kilograms per square inch (kpsi)for a few seconds. The target was removed from the die and then sinteredin an oven to fully form the individual superconducting materials and tobond the materials into a solid target. The target was ramped at 4° C.per minute to 900° C. and held for 25 hours in an oxygen atmosphere. Itwas then ramped down to 400° C. and held for 25 hours, ramped back up to925° C. and held for 25 hours, then ramped down to 400° C. and held foran additional 75 hours. After the latter step, the sample was allowed tofurnace cool (i.e., cool down by simply turning off the furnace) to roomtemperature.

EXAMPLE 6

A film was deposited upon on an IBAD-YSZ coated Hastelloy metalsubstrate using the target from Example 5. The film thickness was about5000 Angstroms and the T_(c) was 92.9 K and a transition temperaturewidth of 0.5 K.

Although the present invention has been described with reference tospecific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent that they are included in the accompanying claims.

1-33. (canceled)
 34. A multilayer thin film structure having alternatinglayers of a first composition and a second composition, a pair of saidalternating layers defining a bi-layer wherein said thin film structureincludes at least 20 bi-layers per micron of thin film such that anindividual bi-layer has a thickness of less than about 50 nanometers.35. The multilayer thin film structure of claim 34 wherein saidbi-layers include a layer of YBCO and a layer of EuBCO.